When a new macronucleus develops from the fertilization nucleus in T. thermophil the micronuclear rDNA sequence is excised, made into a palindromic linear plasmid, and differentially amplified. The ultimate objective of this project is to understand the mechanism of this developmentally programmed somatic DNA alteration. This project exploits the experimental versatility of Tetrahymena for genetic and molecular studies of the mechanism of rDNA maturation. The specific objectives of this project are: 1) To isolate lethal mutations that specifically block the formation of the mature (palindromic) form of the rDNA during macronuclear development. 2) To characterize these mutations molecularly, in order to determine a) what step in the maturation pathway they block, and b) their nature and location. 3) To characterize the mutations genetically, in particular with respect to location and cis- vs. trans-action. 4) To isolate suppressor mutations that restore rDNA maturation to the primary mutants, and to characterize them genetically and molecularly. This project is significant in a variety of contexts: 1) It serves as a model system for the study of developmentally programmed DNA alterations involved in cell differentiation. 2) Extensive DNA alterations and chromosome fragmentations, regularly occurring during macronuclear development, are a way of life among Ciliates; the maturation of Tetrahymena rDNA provides an excellent opportunity for their study. 3) The genetic methodology developed here, coupled with some fortuitous twists of Tetrahymena biology, provide a powerful system for the study of mutations that affect structure/function of the large ribosomal RNA's (18 and 28 S).